Time-Temperature Integrating Indicators have been disclosed in co-pending U.S. application Ser. No. 469,851, filed on May 14, 1974 of Larsson, now a U.S. Pat. No. 3,946,611. The device disclosed in that application incorporated herein by reference, includes an elongated envelope of upper and lower walls of gas impervious material which are heat sealed together in a continuous course extending around the periphery of each, with the walls further having a traverse cross seal intermediate the ends of the envelope so as to define a first and second compartment in the envelope. A gas generating material is disposed in the first compartment and a wick means extends from the first compartment to the second compartment and through the cross seal, whereby the wick means is the only gas communication between the first and second compartment. Further, an indicator composition is deposited on the wick with the indicator composition producing a color change in the presence of a gas generated by the gas generating material. The function of the device is to provide a temperature history, e.g. of a product associated with the device in the visual display as a color front of the indicator wick with the distance of the front advancement being a function of the temperature-time interval, the visual display serving to inform a user whether or not the product has been unduly subjected to adverse temperature conditions as might affect the ultimate product usage. The device is particularly suited for providing an indication of conditions of foods, films, pharmaceuticals, biological preparations and similar products to give indication of decomposition, deterioration or changes of the such composition or products.
Our co-pending application, Ser. No. 515,165, filed on Oct. 16, 1974 describes an improved method of forming the seal about the periphery of the device in such a manner as to eliminate the need for the cross seal. The purpose of the cross seal is to prevent spurious random gas molecule transport within the Time-Temperature Integrating Indicator Device, which if not prevented, would give erroneous readings. Our U.S. application Ser. No. 515,165 now U.S. Pat. No. 3,932,134 incorporated herein by reference teaches a structure which eliminates such spurious random gas molecule transport as a factor in the operation of the device by providing a longitudinally disposed gas barrier means extending between the indicator envelopes upper and lower walls immediate the adjacent longitudinal margin of the wick and along substantially the full length of the wick. If any gas should bypass the ends of the wick nearest the gas source, the spaces between the barrier and wick's side margin is sufficiently small to insure that the gas will not travel longitudinally of the wick to any appreciable or significant distance before it is caused to come into contact with the wick. Thus substantially all the gas absorbed by the wick is caused to make first contact with the wick at or right adjacent to the end of the wick closest to the gas source.
Both of the prior art devices require the use of a rate controlling film in order to control the rate of flow of the gas generating means from the first compartment, which encloses the gas generating means and the second compartment in which is contained the wicking means. The rate at which gas permeates through the rate controlling film is temperature dependent and will determine the response of the device to changes in temperature and time. This rate controlling film as described in the prior art devices is sealed either thermally or adhesively between the upper and lower walls of the device. Such construction causes difficulties in manufacture and assembly of the device.